Storms, Floods, and the Science of Atmospheric Rivers

Eos, Vol. 92, No. 32, 9 August 2011
EOS, Transactions, American Geophysical Union
Volume 92 number 32
9 AUGUST 2011
pages 265–272
Storms, Floods, and the Science
of Atmospheric Rivers
PAGES 265–266
Imagine a stream of water thousands of
kilometers long and as wide as the distance
between New York City and Washington,
D. C., flowing toward you at 30 miles per
hour. No, this is not some hypothetical physics
problem—it is a real river, carrying more
water than 7–15 Mississippi Rivers combined.
But it is not on land. It’s a river of water
vapor in the atmosphere.
Atmospheric rivers (ARs) are narrow corridors
of water vapor transport in the lower
atmosphere that traverse long swaths of the
Earth’s surface as they bind together the
atmospheric water cycle (Figure 1). The characteristic
(indeed defining) dimensions of
these ARs are (1) integrated water vapor
(IWV) concentrations such that if all the
vapor in the atmospheric column were condensed
into liquid water, the result would be
a layer 2 or more centimeters thick; (2) wind
speeds of greater than 12.5 meters per second
in the lowest 2 kilometers; and (3) a shape
that is long and narrow, no more than 400–
500 kilometers wide, and extending for thousands
of kilometers, sometimes across entire
ocean basins.
Research during the past decade has documented
the importance of ARs to the overall
workings of the midlatitude global water
cycle. Moreover, their presence and characteristics
are at the root of the most extreme
precipitation and flooding in areas where
these features encounter mountains. At the
same time, ARs make important contributions
to how much snow and water will be
available in these regions. Thus, understanding
their behavior may be the key to determining
how changing climate patterns influence
extreme precipitation and floods. Overall,
the need to understand ARs opens up a
new set of grand challenges for water cycle,
water supply, and flood prediction science.
Observations of Atmospheric Rivers
Zhu and Newell [1998] helped coin
the term “atmospheric river” based on its
narrow ness and importance to the water
By F. M. Ralph and M. D. Dettinger
cycle. They found that at any given time, an
average of more than 90% of the total poleward
atmospheric water vapor transport
through the middle latitudes is concentrated
in four to five narrow regions that total less
than 10% of the circumference of the Earth
at that latitude. These features are generally
located in the warm sectors of midlatitude
cyclones, ahead of cold fronts. They
continually form, move, and evolve with
storms in the midlatitude storm tracks, sometimes
drawing tropical water vapor and heat
directly into the middle latitudes [e.g., Stohl
et al., 2008; Ralph et al., 2011].
Since the seminal work of Zhu and Newell
[1998], the prevalence and role of ARs in the
water cycle and in continental weather have
become ever more clear, partly because of
the advent of microwave remote sensing
from polar- orbiting satellites, especially the
Special Sensor Microwave Imager (SSM/I),
which provides frequent global measurements
of IWV over the Earth’s oceans, measurements
that previously were available
above only the relatively few sites where
weather balloons and related instruments
were deployed. The imager works very well
over oceans and, since its spatiotemporal
coverage became adequate in about 1998,
has focused growing attention on ARs (see
Figure 1a) in ways that previous water vapor
data could not.
In the years since then, a growing number
of field experiments and related studies have
Fig. 1. Analysis of an atmospheric river (AR) that hit California on 13–14 October 2009. (a) A
Special Sensor Microwave Imager (SSM/I) satellite image from 13–14 October showing the AR
hitting the California coast; color bar shows, in centimeters, the amount of water vapor present
throughout the air column at any given point if all the water vapor were condensed into one
layer of liquid (vertically integrated water vapor). (b) Rain gage data for 12:00 UTC on 14 October
2009 showing the total amount of precipitation (in inches) that occurred over the previous
24 hours. (c) Discharge for Nacimiento River (site indicated by red triangles in other panels);
data are from U.S. Geological Survey stream gage 11148900. (d) Statewide streamflow historical
ranking of 14 October 2009, compared to discharges on the same day of the year recorded by
gages with more than 30 years of data.

explored the physical characteristics and
effects of ARs, focused mostly over the eastern
Pacific Ocean and western North America
(a bibliography of AR- related research
papers and many additional resources on
ARs are available at http://​www​.esrl​.noaa​
.gov/​psd/ atmrivers/). Research aircraft observations
in two ARs above the eastern North
Pacific in the winters of 1998 and 2005 [Ralph
et al., 2005, 2011] showed that they transported
water vapor at about 13–26 cubic
kilometers per day, a rate equivalent to 7.5–
15 times the average daily discharge of the
Mississippi River into the Gulf of Mexico.
A Closer Look at Rainfall
From Atmospheric Rivers
Because ARs transport so much water
vapor, they represent a significant source of
precipitation to coastal regions. For example,
a recent numerical model study [Smith
et al., 2010] estimated that roughly 20–40%
of the water vapor transported ashore by
an AR crossing over northern California
was rained out there. This rainout happens
because when ARs make landfall on the
West Coast of North America (as well as on
other continents [e.g., Stohl et al., 2008]),
they are forced up and over coastal mountains,
where they cool and condense large
parts of their heavy burden of vapor [e.g.,
Neiman et al., 2008; Leung and Qian, 2009].
In a recent example, an AR event produced
more than 410 millimeters (16.5
inches) of rainfall at one site in coastal California
on 14–15 October 2009 (Figure 1).
This particular AR had a very long fetch,
spanning most of the North Pacific (Figure
1a), and upon making landfall deposited
more than 200 millimeters of rain along
a swath of coastal California several hundred
kilometers wide (Figure 1b). Significant
streamflow resulted, including a 5- meter rise
in water level on the Nacimiento River over
12 hours (Figure 1c), with the flows cresting
at 525 cubic meters per second (18,600
cubic feet per second). Record- high daily
streamflows (for that date of year) were also
observed at many other stations in central
and northern California (Figure 1d).
It should be noted that this peak flow of
the Nacimiento River exceeded the annual
peak flow in 28 of the past 40 years and did
so in spite of the very dry conditions preceding
this storm. This event exhibits key attributes
found in other extreme ARs [e.g., Neiman
et al., 2008; Ralph et al., 2011], including
very large IWV values, indications of entrainment
of tropical water vapor (from the western
Pacific in this case, incorporating remnants
of a western Pacific typhoon), and the
fact that it stalled over parts of the West Coast
in ways that amplified the storm’s impacts.
Historically, AR storms have been the
sources of many (and, in some areas, most)
floods in the Pacific coast states. For example,
all storms that resulted in declared
flood conditions on the Russian River of central
California from 1998 to 2005 arose from
Eos, Vol. 92, No. 32, 9 August 2011
landfalling ARs [Ralph et al., 2006]; similar
relations appear to exist between ARs and
major flooding in most rivers from California
to Washington State. In addition to their
roles as producers of extreme storms and
flood hazards, it is important to mention that
ARs also are major contributors to western
(especially California) water supplies [Dettinger
et al., 2011; Guan et al., 2010]. Indeed,
the half dozen or so ARs per year that make
landfall in California have contributed an
average of one third to one half of all the
state’s precipitation, with a single typical AR
storm yielding perhaps 2.5–5 cubic kilometers
of precipitation, or roughly 10% of the
annual average runoff of the entire Sacramento
River basin.
Studies of Atmospheric Rivers
From the West Coast
The dual roles of ARs as hazards and
water resources in many coastal regions may
become a more pressing issue under anthropogenic
climate change, which may alter
both hazardous and productive aspects of
these storms [Dettinger, 2011]. For example, in
view of the havoc that these storms wreak on
the Pacific coast states, understanding and
predicting them on time scales from days to
decades, and at spatial scales from mountain
ranges like the Sierra Nevadas and Cascades
to individual river basins, present major challenges
for West Coast meteorologists, climatologists,
and hydrologists. Although research
to address the roles of ARs elsewhere is
mostly just beginning, AR research has been
vigorous and productive on the West Coast
for more than a decade.
Over the past decade several studies led by
the National Oceanic and Atmospheric Administration
(NOAA) (see http://​hmt​.noaa​.gov/)
have explored the inner workings of ARs and
the effects they produce, through intense field
campaigns and the use of new meteorological
and hydrometeorological sensors including
radar and sounding assets, research aircraft,
and other remote sensing tools as well
as numerical models. As understanding of the
scales and impacts of ARs has grown, scientific
efforts have expanded to include other
agencies on federal, state, and local levels,
including the U.S. Geological Survey (USGS),
the U.S. Army Corps of Engineers, NASA, California’s
Department of Water Resources and
the California Energy Commission, and local
agencies around San Francisco Bay and in
fire- scarred areas of southern California. By
now, a wide- ranging collection of studies are
currently under way on the West Coast to elucidate
various aspects of AR phenomena and
their impacts on the West Coast.
One study is the Hydrometeorology
Testbed- West (HMT- West), led by the Physical
Sciences Division of NOAA’s Earth
System Research Laboratory. HMT- West
includes long- term geographically focused
field research, as well as innovative monitoring
and modeling to improve scientific
understanding and short- term prediction
of extreme precipitation events and flooding
associated with ARs. Efforts have been
focused around the Russian and American
river basins of central California.
Another is the Enhanced Flood Response
and Emergency Preparedness (EFREP)
program led by California’s Department of
Water Resources, NOAA, and Scripps Institution
of Oceanography. EFREP seeks to foster
development and deployment of statewide
monitoring, modeling, and decision
support programs that make key findings
from HMT- West operational, for better detection,
monitoring, and prediction of ARs and
their impacts. A key component is a “picket
fence” of four evenly spaced coastal observatories
to monitor ARs, statewide observational
networks of soil moisture and IWV,
and 10 snow- level radars, all with associated
decision support capabilities.
The CalWater project, led by the California
Energy Commission, NOAA, and Scripps,
completed a major field campaign last winter
that is providing data for research initiatives
to address details of the interactions of
ARs with topography, aerosols, and air pollution.
Another goal is to critically assess ARs
in climate models to quantify several key
uncertainties in climate projections of precipitation
for California [Dettinger, 2011].
A major project led by the USGS Multi-
Hazards Demonstration Project, called
ARkStorm, has developed a storm emergency
scenario being used in hazards assessments
and activities aimed at improving emergency
preparedness and public awareness of the
potential for catastrophic AR storms in California.
Their scenario, based on the most
recent AR science, rivals the largest storms
and floods in California’s history and allows
researchers to explore possible responses to
historic levels of flooding, landslides, wind
damage, water pollution, and attendant infrastructure
and economic disruptions.
Outside of California, in 2009 a major
storm damaged an Army Corps of Engineers
dam near Seattle that protects a heavily
developed area from flooding. After that,
dam operators could not use the full flood
storage capacity of the reservoir (although
repairs have now restored much of this
capacity). To mitigate the risk of flood damages
from 2009 to 2011, NOAA and the
Corps applied concepts and tools for better
monitoring of ARs that had been developed
in California. AR-related observations
were deployed to Washington State to provide
actionable information on AR storms
approaching the area above the dam.
Finally, the Winter Storms and Pacific
Atmospheric Rivers (WISPAR) project, jointly
led by NOAA, NASA, and Northrop Grumman,
performed field experiments in early
2011 using a high- altitude, long- endurance
drone aircraft, the Global Hawk, to make offshore
observations of several ARs over the
Pacific Ocean. The field campaign included
deploying a newly developed dropsonde system
to document in detail the structure of
water vapor transport in ARs.

Details of these and other investigations
are available at http://​www​.esrl​.noaa​.gov/​
psd/​ atmrivers/.
A Scientific Challenge
As is illustrated here, and as was highlighted
in a special session on ARs at the
2010 AGU Fall Meeting (see http://​hmt​.noaa​
.gov/​news/​2011/​012511​.html), ARs have
become a focus of research and development
aimed at better physical understanding,
monitoring, short- term forecasts
and warnings, and climate projections.
Recognizing ARs is key to forecasting
extreme precipitation and flooding in the
Pacific coast states and is now included
in forecaster training by NOAA and other
agencies.
Because of the vast amounts of water that
they transport and deliver, ARs are probably
just as important in many other regions
of the globe where they have been less studied.
For example, recent flood catastrophes
in Nashville, Tenn. (May 2010), and North and
South Carolina (October 2010) were associated
with ARs making landfall from over the
Gulf of Mexico and Atlantic. The central role
of ARs in water cycle dynamics outside the
tropics, increasing pressure on limited water
resources, and changing exposures to flood
risks due to development and climate changes
all demand improved scientific understanding
and forecasts of ARs. Providing those improvements
makes ARs a grand challenge for water
Eos, Vol. 92, No. 32, 9 August 2011
cycle science, with important implications for
flooding and water supply.
References
Dettinger, M. (2011), Climate change, atmospheric
rivers, and floods in California—A multimodel
analysis of storm frequency and magnitude
changes, J. Am. Water Resour. Assoc., 47(3),
514–523, doi:10.1111/​j.1752-1688​.2011​.00546.x.
Dettinger, M. D., F. M. Ralph, T. Das, P. J. Neiman,
and D. R. Cayan (2011), Atmospheric rivers,
floods and the water resources of California,
Water, 3(2), 445–478, doi:10.3390/​w3020445.
Guan, B., N. P. Molotch, D. E. Waliser, E. J. Fetzer,
and P. J. Neiman (2010), Extreme snowfall
events linked to atmospheric rivers and surface
air temperature via satellite measurements,
Geophys. Res. Lett., 37, L20401, doi:10.1029/​
2010GL044696.
Leung, L. R., and Y. Qian (2009), Atmospheric rivers
induced heavy precipitation and flooding in
the western U.S. simulated by the WRF regional
climate model, Geophys. Res. Lett., 36, L03820,
doi:10.1029/​2008GL036445.
Neiman, P. J., F. M. Ralph, G. A. Wick, J. Lundquist,
and M. D. Dettinger (2008), Meteorological characteristics
and overland precipitation impacts
of atmospheric rivers affecting the West Coast
of North America based on eight years of SSM/I
satellite observations, J. Hydrometeorol., 9(1),
22–47, doi:10.1175/​2007JHM855.1.
Ralph, F. M., P. J. Neiman, and R. Rotunno (2005),
Dropsonde observations in low-level jets over the
northeastern Pacific Ocean from CALJET-1998
and PACJET-2001: Mean vertical- profile and
atmospheric- river characteristics, Mon. Weather
Rev., 133(4), 889–910, doi:10.1175/​MWR2896.1.
Ralph, F. M., P. J. Neiman, G. A. Wick, S. I.
Gutman, M. D. Dettinger, D. R. Cayan, and
A. B. White (2006), Flooding on California’s
Russian River: Role of atmospheric
rivers, Geophys. Res. Lett., 33, L13801,
doi:10.1029/2006GL026689.
Ralph, F. M., P. J. Neiman, G. N. Kiladis, K. Weickmann,
and D. W. Reynolds (2011), A multiscale
observational case study of a Pacific
atmospheric river exhibiting tropical- extratropical
connections and a mesoscale frontal
wave, Mon. Weather Rev., 139(4), 1169–1189,
doi:10.1175/​2010MWR3596.1.
Smith, B. L., S. E. Yuter, P. J. Neiman, and D. E.
Kingsmill (2010), Water vapor fluxes and orographic
precipitation over northern California
associated with a land- falling atmospheric
river, Mon. Weather Rev., 138(1), 74–100,
doi:10.1175/2009MWR2939.1.
Stohl, A., C. Forster, and H. Sodemann (2008),
Remotes sources of water vapor forming precipitation
on the Norwegian west coast at 60°N:
A tale of hurricanes and an atmospheric river,
J. Geophys. Res., 113, D05102, doi:10.1029/​
2007JD009006.
Zhu, Y., and R. E. Newell (1998), A proposed
algorithm for moisture fluxes from atmospheric
rivers, Mon. Weather Rev., 126(3), 725–735,
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Author Information
F. M. Ralph, Physical Sciences Division, Earth System
Research Laboratory, NOAA, Boulder, Colo.;
E- mail: marty.ralph@​noaa​.gov; and M. D. Dettinger,
USGS and Scripps Institution of Oceanography,
La Jolla, Calif.
New Education and Outreach Opportunities
for Scientists
PAGES 266–267
A scientist works late to finish up yet
another proposal for research funding. Time is
short—the proposal is due in only a week. The
research description is well in hand, compelling
and at the forefront of the field. But the scientist
is less confident of what to propose for
a “broader impacts” component that will actually
be meaningful. What does it mean to have
a broader impact? What can be proposed that
will make a difference but will not divert too
much time from conducting research, searching
for funding, or writing papers?
For many scientists, particularly those
who rely on soft money for research funding,
the above scenario is a familiar story.
These days, research solicitations from
funding agencies consistently require that
in addition to proposing innovative and
cutting-edge research, scientists must also
include elements in their proposals that provide
meaningful broader impacts to their
research programs—in essence, they must
show how their research will benefit society
and spread knowledge.
To help scientists, research programs,
and organizations tackle this part of their
grant proposals, the National Earth Science
Teachers Association (NESTA; http://​www​
. nestanet​.org) is offering a number of opportunities
that can help bring new research to
teachers, students, and the public. Through
these opportunities, new and dynamic science
can reach a broad population without
forcing researchers to build outreach programs
from scratch.
Maximizing Outreach Efforts
Through NESTA
Grant requirements vary in the types of
activities that qualify for outreach elements,
and they can range from providing undergraduate
research opportunities to working
with K–12 teachers or reaching out to the
community through informal educational
organizations or events. The challenge for
many scientists seeking to undertake K–12
or public outreach activities is finding a way
to provide meaningful broader impacts that
actually reach significant numbers of people.
While developing a new Web site to share science
can be creative and enjoyable, experience
proves that it is very difficult to draw
attention to Web-based resources in the vast
maze that is the Internet today unless the
resources are linked to or made available on
a Web site already heavily used by the audience
the scientist is trying to reach. Likewise,
while visiting a classroom in a local primary
or secondary school can be very rewarding
for all involved, many scientists would like to
have opportunities to have an effect on larger
numbers of teachers and students.
Scientists naturally have limited amounts
of time they can devote to outreach elements
while also pursuing their demanding
research and other professional responsibilities.
Reaching a large audience, though,
requires substantial effort, and the scientist
does not want to waste his or her time. Considering
the small amount of funding from
grants that scientists can typically apply
to these activities, coupled with the critical
importance of science to society, it is
imperative that scientists find effective and
efficient approaches for public outreach
through research projects that magnify the
effects of their efforts.
Through NESTA, scientists do not have to
look far to maximize their outreach efforts. As
a 501(c)(3) tax-exempt professional society,
founded in 1983 with a mission to facilitate
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science education, NESTA directly serves the
K–12 Earth and space science educator community
nationally as well as through affiliate
organizations working at the state level.
NESTA recently became the host of Windows
to the Universe (W2U; http://​www​
. windows2universe​.org), an Earth and space